On the asymmetry of Stokes profiles

We find all the sources of asymmetries in Stokes profiles available for a unidimensional line of sight. The analytical solution for the transfer equation for polarized light obtained by Lopez Ariste & Semel is used for this purpose. Three sources of asymmetries are obtained: the first is the well-known case of depth-dependent velocity fields; the two others are due to wavelength asymmetries in the absorption matrix and the emission vector. These emission vector asymmetries can be produced by the presence of a polarized (but symmetric) source function. Some formulae are derived describing the asymmetries owing to depth-dependent velocity fields in Stokes V for the case of the Zeeman effect in a multiplet, as well as for the net area of the Stokes V profile, the so-called net circular polarization (NCP). An implication of this last formula is that Milne-Eddington atmospheric models, even when generalized to velocity and magnetic fields that vary linearly with optical depth, cannot account for the observed NCPs.